Method of manufacturing an immersion member with pore-sealing layer

ABSTRACT

A manufacturing method for immersion members for molten metal baths, wherein a thermal sprayed coating including 1-50 wt % of tungsten boride, 3-25 wt % of one or more of Ni, Co, Cr, and Mo as a metal phase, and a remainder including tungsten carbide, is formed on the surface of a immersion member for use in molten metal baths, and subsequently, impregnation processing is conducted with respect to the thermal sprayed coating in a processing fluid having as a main component thereof chromic acid (H 2  CrO 4  and H 2  Cr 2  O 7 ), and subsequently, baking processing is conducted. In accordance with this manufacturing method, a surface layer possessing fine microstructure and high bond strength not conventionally available is provided, and it is possible to manufacture a superior immersion member for use in molten metal baths which has superior resistance to corrosion, resistance to corrosive peeling, and resistance to abrasion, and to which metals do not easily adhere.

TECHNICAL FIELD

The present invention relates to a manufacturing method for immersionmembers immersed for a long period in a high temperature molten metalbath such as one of molten zinc, molten aluminum, molten tin, and thelike. In particular, the present invention relates to a manufacturingmethod for immersion members for use in molten metal baths in moltenzinc plating production lines, molten aluminum plating production lines,molten tin plating production lines, or the like; for example, sinkrolls and support rolls which are used in an immersed state in a moltenzinc plating bath or a molten aluminum plating bath.

BACKGROUND ART

It is apparent that a resistance to corrosion resulting from moltenmetals is in great demand with respect to immersion members which areused over a long period of time in an immersed state in high temperaturemolten metal baths such as one of molten zinc, molten aluminum, ormolten tin, or the like. In particular, in sink rolls and support rolls,it has been desirable not merely that resistance to corrosion resultingfrom molten metals be present, but also that abrasion resulting from thecontact between the roll and the substrate to be plated, such as a steelplate or the like, which is immersed in the bath, be unlikely to occur,and that adhesion of metals also be unlikely to occur.

When metal adhesion occurs on immersion rolls such as sink rolls,support rolls or the like, damage is caused to the substrate to beplated, or to the plating surface of the steel plate or the like, whichis guided by these rolls and immersed in the bath. Furthermore, for thisreason, immersion rolls such as sink rolls and support rolls have becomeunsuitable for use.

Conventionally, in response to these varying demands, immersion membershaving various cermet materials thermal sprayed thereon have beendeveloped and used; however, such members are as yet insufficient. Forexample, a WC-Co cermet thermal sprayed coating is used as an immersionmember for use in molten metal baths; however, such a member isinsufficient from the point of view of molten metal corrosionresistance.

Furthermore, the above-described demands have become more and moreincreasing in concert with demands for increasing quality of platedproducts, demands for a reduction in manufacturing costs, and demandsfor extended service life of immersion rolls.

In response to these demands, the present inventors previously inventedan immersion member for use in molten zinc baths and the like, in whichthe surface coating of the immersion member itself comprises one or moreof tungsten carbides, tungsten borides, and molybdenum borides, inaddition to Co, and this was disclosed in Japanese Patent ApplicationHei 1-231293 (Japanese Patent Application, Laid-Open No. Hei 3-94048,laid open date: Apr. 18, 1991). Corrosion resistance of the immersionmember with respect to molten metal baths was achieved by means of thisinvention; however, there was a problem in that corrosive peelingoccurred during use over a long period of time.

In general, cracks and micropores are present in a thermal sprayedcoating. During use of an immersion member in a molten metal bath over along period of time, the molten metal penetrates to the interior of thethermal sprayed layer through these cracks and micropores and breaksdown the thermal sprayed coating, corroding this thermal sprayed coatingfrom below the surface, so that a phenomenon is noted in which thethermal sprayed coating peels away. This is termed corrosive peeling.

In order to solve this problem, the present inventors tested immersionmembers in which the cracks and micropores present in the thermalsprayed coating are filled with coal tar; however, under the conditionsof high temperature present in the molten metal baths, the organicsubstances present in the coal tar decomposed and became gassified, andfor this reason, the quality of the thermal sprayed coating wasdeteriorated, so that an immersion member having a long service lifecould not be obtained. Furthermore, the gas produced by thedecomposition of the organic substances in the molten metal bathproduced undesirable effects.

Furthermore, in order to avoid this phenomenon, an attempt was made tosubject the immersion member to heat processing immediately prior to usein the molten metal bath after filling the cracks and micropores of thethermal sprayed coating of the immersion member for use in molten metalbaths with coal tar; however, gas was produced by the decomposition ofthe organic substances contained in the coal tar during heat processing,and for this reason, micropitting was produced, and the coal tar fillingmaterial itself was lost, so that the desirable properties could not beobtained.

DISCLOSURE OF THE INVENTION

In order to solve the problems described above, the present inventorshave conducted extensive research as described above, and as a result ofthis research, the present invention has been made.

First, an important feature of the present invention is the addition, inthe thermal sprayed coating composition, of tungsten borides (WB and thelike), to produce a Cr₂ O₃ -B₂ O₃ system glass in at least the cracksand micropores, by means of an oxidation reaction with H₂ CrO₄, or thelike, and to form a fine and strong thermal sprayed pore-sealing layerusing this effect. In accordance with the present invention, it ispossible to obtain a superior immersion member for use in molten metalswhich is provided with a fine and strong surface film layer not found inthe conventional art.

Hereinbelow, the present invention will be explained in detail.

Conventionally, a WC-Co cermet was employed in immersion members for usein molten metal baths; however, as a result of the research of thepresent inventors, it was determined that, in addition to WC, WB issuperior from the point of view of corrosion resistance in molten metal.Next, it was determined that WB has a higher coefficient of thermalexpansion and that the resulting thermal sprayed coating has a strongerthermal shock resistance than that of WC. Furthermore, it was determinedthat in an oxidizing atmosphere, borides form B₂ O₃ on the surfacethereof, and that at high temperatures, a portion of this B₂ O₃ isvolatilized; however, a certain amount remains on the surface.

Furthermore, the present inventors have determined that it is possibleto obtain a superior coating when a thermal spraying material consistingof a cermet in which WC and WB are combined with at least one of Ni, Co,Cr, and Mo to coat WC and WB with Ni, Co, or the like, or a thermalspraying material consisting of WC and WB which are agglomerated with atleast one of Ni, Co, Cr, and Mo and are subjected to granulation, and issintered in a neutral atmosphere, these materials being subjected tothermal spray by a high-velocity oxygen fuel gun method or a plasmaspraying method. The coating contains unavoidable impurities.

Furthermore, WB-WC is superior to WC in molten metal wettability, sothat adhesion is unlikely to occur with respect to, for example, moltenzinc. However, it was discovered that when the amount of WB addedbecomes large, satisfactory thermal spraying becomes difficult in astandard atmosphere.

Accordingly, it is preferable that the limitation on the amount of WBcontained in the thermal sprayed coating be set to less than 50 weight%. Furthermore, when the amount thereof is too small, the desiredeffects cannot be realized. Accordingly, the amount of WB containedshould be within a range of 1-50 weight %. It is more preferable thatthe amount contained be within a range of 10-40 wt %. Instead of using1-50 wt % of tungsten boride, the thermal sprayed coating may contain1-49 wt % of tungsten boride and 1-30 wt % of one or more of chromiumboride, molybdenum boride, zirconium boride and titanium boride, whereinthe total amount of these metal borides is less than 50 wt %.

The reason for the addition of at least one of Ni, Co, Cr, and Mo as ametal phase is to increase resistance to peeling, and to increasehardness, so that superior layer may be obtained. The amount of at leastone of Ni, Co, Cr, and Mo should preferably be within a range of 3-25 wt%. At amounts of less than 3 wt %, no cermet effects can be obtained.Furthermore, when the metal phase exceeds 25 wt %, the effect of addingceramics which are WC, WB or the like is lost. If at least one of Cr andMo is added in an amount of less than 15 wt %, it is possible to improvethe molten metal corrosion resistance of the metal phase. It istherefore necessary to limit the total amount of Ni, Co, Cr, and Mo toless than 25 wt %.

The immersion member for use in molten metal baths is subjected tosurface polishing after thermal spraying; in the manufacturing method ofthe present invention, it is possible to conduct final polishing afterthermal spray coating, prior to processing fluid impregnationprocessing, or after baking processing. A strong acid solution in whichchromic acid is included as a main component is used as the processingfluid. In order to conduct the impregnation of the processing fluid intothe thermal sprayed coating, it is possible to immerse the member foruse in molten metal baths and having formed thereon the thermal sprayedcoating, into the processing fluid, or to brush the processing fluidonto the thermal sprayed coating formed on the surface of the member foruse in molten metal baths. By means of the impregnation processing, theprocessing fluid penetrates the cracks and micropores, and it is thuspossible to fill these cracks and micropores. Next, by means of theinitial heating during baking, the chromic acid (H₂ CrO₄ and H₂ Cr₂ O₇)present in the processing fluid within the cracks and micropores isconverted to CrO₃ to fill these cracks and micropores results. Thechromic acid solution is desiccated by means of the heating, and themoisture component thereof is removed; however, if heating is continued,in the vicinity of 200° C., CrO₃ (chromic acid anhydride) melts, and itis possible to conduct CrO₃ molten salt processing in the thermalsprayed coating. The thermal sprayed coating in contact with this isoxidized, and the CrO₃ is finely bonded with the thermal sprayedcoating. That is to say, by means of the reaction using CrO₃, the Cr₂ O₃which is formed and the inner surfaces of the cracks and micropores arechemically bonded, and a fine ceramic-filled thermal sprayed coating isformed. The baking temperature should preferably be greater than 400°C., at which temperature Cr₂ O₃ conversion can be sufficientlyconducted, and less than 500° C.; at these temperatures, almost all CrO₃is converted to Cr₂ O₃.

Furthermore, it has been determined that the reason that the immersionmember produced in accordance with the present invention exhibitssuperior corrosion resistance with respect to molten metals is that,after the impregnation processing with processing fluid and bakingprocessing, the borides, such as WB and the like, which are present inthe thermal sprayed coating are finely and strongly bound with Cr₂ O₃.

In particular, in the present invention, the vitrification reaction ofthe B₂ O₃ produced by the oxidation of the borides present in thethermal sprayed coating and the CrO₃ is important. That is to say, thevitrification of B₂ O₃ begins at a temperature of approximately 300° C.during heating; however, at this temperature, CrO₃ becomes a moltenoxide, and the vitrified B₂ O₃ and the CrO₃, which has become a moltenoxide, oxidize the surface of the thermal sprayed coating and the layerwithin the cracks and micropores, so that fine fusion occurs so as toproduce a CrO₃ -Cr₂ O₃ -B₂ O₃ glass substance. Furthermore, when heatingis continued and the temperature reaches a level above 400° C., the CrO₃is converted to Cr₂ O₃ and solidifies completely; however, the B₂ O₃component becomes softer, a portion thereof reacts with the Cr₂ O₃ tobecome more finely bound thereto, and the cracks and micropores arefilled. The melting point of B₂ O₃ is approximately 450° C.

Accordingly, the combination of the thermal sprayed coating and theprocessing of the present invention should be termed "glass sealing",and the oxide bonds between the thermal sprayed coating and CrO₃, andthe bond resulting from vitrification of CrO₃ and B₂ O₃ produce combinedfunction to provide a strong and complete crack-and-micropore-fillingeffect, as well as an effect of an increase in layer bonding, isexhibited. Furthermore, no volatilization or combustion of the moisturecomponent or alcohol component occurs during the thermal reaction (inthe present invention, a dehydration reaction occurs; however, themoisture component is removed prior to the formation of molten CrO₃),and there is no formation of micropitting during heating. For thisreason, it is thought that a fine and strong surface layer can beformed.

Furthermore, heating to a temperature in excess of 500° C. producesstrain or residual stress in immersion members for use in molten metalbaths, so that such heating is not preferable.

As a result of the above, it is recommended that the heating temperatureduring baking processing be within a range of 400° C. to 500° C.

Furthermore, a strongly acidic fluid comprising primarily chromic acidis used as the impregnation processing fluid of the present invention;and the addition of Na⁺ and K⁺ ions may improve the permeability of thisfluid and apply the solubility of the metallic oxides on the surface ofthe layer to B₂ O₃, and a small amount of the salts thereof may beadded. For example, a small amount of sodium hydroxide (NaOH) orpotassium hydroxide (KOH) may be added.

Furthermore, it is possible to add sodium molybdate or ammoniummolybdate, or both sodium molybdate and ammonium molybdate, to theprocessing fluid 3. By means of this, the vitrification described aboveis improved, and furthermore, as a result of the presence of MoO₃, it ispossible to obtain a finer and stronger bonding and diminution effect ofmicropores and increasing fineness of layer's microstructures. This isthought that the components filling the cracks or micropores form a Cr₂O₃ -B₂ O₃ -MoO₃ -borate system compound (for example, Na₂ B₄ O₇).

Furthermore, it is also possible to blend a water-soluble coating agent;however, in this case, an oxidation reaction is carried out by means ofchromic acid, so that such an agent should be blended immediately priorto the use thereof in the impregnation processing.

In order to increase the reliability of the coating and strengtheningeffects of the thermal sprayed coating resulting from the manufacturingmethod of the present invention, it is also possible to repeat the cycleof the processing fluid impregnation processing and baking processingtwo or more times.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinbelow, an embodiment of the present invention will be explained.

EMBODIMENT I

A plurality of metal plates conforming to American Iron and SteelInstitute standard AISI 316 (corresponding to the JIS standard SUS 316)having a thickness of 5 mm, a width of 30 mm and a length of 100 mm wasprepared, and on one side of each metal plate, a thermal sprayed coatingwas formed by means of a high velocity oxygen fuel gun method, and asshown in Table 1, metal plates having formed thereon thermal sprayedcoating having the compositions a-k, o, p, q, and r were produced. Thecompositions of the thermal sprayed coating formed on the sample metalplate surfaces are shown in Table 1. The compositions having thereference letters a-k fulfill the conditions of the present invention.The compositions referenced o and p do not fulfill the conditions of thepresent invention and are presented as Comparative Examples. The samplemetal plates referenced q and r are Conventional Examples correspondingto standard conventional products; they employ WC-Co system cermetthermal sprayed coating.

Next, as shown in Table 2, impregnation processing in processing fluidand baking processing were conducted on the sample metal plates preparedas described above, and a molten zinc bath immersion test was conducted.In concert with this, a molten zinc immersion test was conducted withrespect to the sample metal plates which had not been subjected toimpregnation processing in processing fluid or baking processing, andcomparison was made with the examples of the present invention.

The plating bath employed in the test was a zinc aluminum (Zn-Al)plating bath containing 3% aluminum. In this test, each sample metalplate was continuously immersed in this plating bath, and the bathtemperature was maintained at 500° C.; the state of the thermal sprayedcoating of each sample metal plate was then visually evaluated. As aresult of this evaluation, those plates which exhibited no corrosivepeeling even after a period of 30 days of continuous immersion areindicated by the designation ⊚, plates which exhibited no corrosivepeeling after 10 days of continuous immersion but which exhibitedcorrosive peeling after 15 days of continuous immersion are indicated bythe designation ◯, while plates which exhibited corrosive peeling aftera period of 10 days of continuous immersion are indicated by thedesignation Δ.

In Table 2, Examples 1-28 correspond to examples of the presentinvention, while Comparative Examples 31-42 are examples having thermalsprayed coating, identical to those of 1-28, which were not subjected toimpregnation processing in the processing fluid or to baking processing.As is clear from the results shown in the Table, even immersion memberspossessing thermal sprayed coating having identical compositions did nothave long service lives if not subjected to impregnation processing inthe processing fluid and baking processing. Furthermore, even ifimpregnation processing in the processing fluid and baking processingwere conducted with respect to immersion members having a conventionalWC-Co cermet thermal sprayed coating formed thereon, satisfactoryeffects could not be obtained, as shown by Comparative Examples 45 and46. Furthermore, as is clear from Comparative Examples 43 and 44, incases in which the metal phase of the thermal sprayed coating was 2 wt %and 38 wt %, these examples were unacceptable in spite of the fact thatWB was contained in an amount of 10 wt %. This was found to be sobecause, in the case in which the metal phase is too small, the ceramicmaterial peels easily away from the thermal sprayed coating, while whenthe metal phase is too large, the metal phase is corroded by the moltenmetal.

From the above Examples,. Comparative Examples, and ConventionalExamples, it was found that the effects of the present invention aregreat.

INDUSTRIAL APPLICABILITY

As stated above, the manufacturing method for immersion members for usein molten metal baths in accordance with the present invention iscapable of producing immersion members for use in molten metal bathswhich possess corrosion resistance with respect to molten metals, havesuperior resistance to corrosive peeling, have superior resistance toabrasion, have a long service life, have superior wettability withrespect to molten metals, and exhibit little metal adhesion, so thatsuch members are extremely useful in industry.

                  TABLE 1                                                         ______________________________________                                        Composition of Thermal Sprayed Coating                                               Ceramic Composition (wt %)                                                                     Metal-Phase                                           Ref-                Other         Composition                                 er-      WB             Bor-        (wt %)                                    ence     (W.sub.2 B.sub.5)                                                                     CrB.sub.2                                                                            ides  WC    Co  Ni  Mo   Cr                           ______________________________________                                        Used  a      10      --   --    Re-   10  --  --   --                         in                              main-                                         Em-                             der                                           bodi- b      10      --   MoB 3 Re-   10  --  3    --                         ments                           main-                                         of                              der                                           Present                                                                             c      20      --   --    Re-   --  13  --   --                         Inven-                          main-                                         tion                            der                                                 d      20      5    ZrB.sub.2 5                                                                         Re-   12  --  --   --                                                         main-                                                                         der                                                 e      20      --   TiB.sub.2 10                                                                        Re-   11  --  --   5                                                          main-                                                                         der                                                 f      20      --   MoB   Re-    8  --  5    --                                                   25    main-                                                                         der                                                 g      20      --   --    Re-    5   7  --   --                                                         main-                                                                         der                                                 h      30      --   --    Re-   10   3  5    --                                                         main-                                                                         der                                                 i      30      5    --    Re-   --  12  3    5                                                          main-                                                                         der                                                 j      30      5    TiB.sub.2 5                                                                         Re-   12   5  3    --                                                         main-                                                                         der                                                 k      40      --   --    Re-   12  --  --   --                                                         main-                                                                         der                                           Com-  o      10      --   --    Re-    2  --  --   --                         para-                           main-                                         tive                            der                                           Exam- p      10      --   --    Re-   35  --  3    --                         ples                            main-                                                                         der                                           Con-  q      --      --   --    Re-   10  --  --   --                         ven-                            main-                                         tional                          der                                           Exam- r      --      --   --    Re-   12  --  --   5                          ples                            main-                                                                         der                                           ______________________________________                                         Note 1: Thermal spraying on one surface of an AISI316 sample having           dimensions of 5 mm × 30 mm × 100 mm                               Note 2: In the Table, (W.sub.2 B.sub.5) indicates that a small amount of      W.sub.2 B.sub.5 is contained in the WB.                                  

                                      TABLE 2                                     __________________________________________________________________________           Thermal                                                                       Sprayed                        Molten Zn                                      Coating Compo-                                                                         Impregnation  Baking  Bath                                           sition (from                                                                           Processing    Proces- Immersion                               No.    Table 1) Fluid         sing    Test                                    __________________________________________________________________________    Embodiments                                                                   of                                                                            Present                                                                       Invention                                                                      1     a        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                       2     a        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                       3     b        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                       4     b        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                       5     c        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                       6     c        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                       7     c        30% Chromic Acid, 2% Am-                                                                    450° C., Baking                                                                ◯                                           monium Molybdate Mixture                                                                    30 minutes                                       8     d        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                       9     d        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      10     e        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                      11     e        30% Chromic Acid, 2%                                                                        450° C. Baking                                                                 ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      12     e        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate, 2% Am-                                                                    30 minutes                                                      monium Molybdate Mixture                                      13     f        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                      14     f        30% Chromic Acid, 2%                                                                        450° C. Baking                                                                 ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      15     f        30% Chromic Acid, 2% Am-                                                                    450° C., Baking                                                                ◯                                           monium Molybdate Mixture                                                                    30 minutes                                      16     g        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                      17     g        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      18     g        30% Chromic Acid, 2% Am-                                                                    450° C., Baking                                                                ◯                                           monium Molybdate Mixture                                                                    30 minutes                                      19     g        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate, 2% Am-                                                                    30 minutes                                                      monium Molybdate Mixture                                      20     h        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                      21     h        30 Chromic Acid, 2%                                                                         450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      22     i        30% Chromic Acid                                                                            450° C.. Baking                                                                ◯                                                         30 minutes                                      23     i        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      Embodiments                                                                   of Present                                                                    Invention                                                                     24     j        30% Chromic Acid                                                                            450° C., Baking                                                                ◯                                                         30 minutes                                      25     j        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      26     k        30% Chromic Acid                                                                            450° C. Baking                                                                 ◯                                                         30 minutes                                      27     k        30% Chromic Acid, 2%                                                                        450° C., Baking                                                                ◯                                           Sodium Molybdate Mixture                                                                    30 minutes                                      28     k        30% Chromic Acid, 2% Am-                                                                    450° C., Baking                                                                ◯                                           monium Molybdate Mixture                                                                    30 minutes                                      Comparative                                                                   Examples                                                                      31     a        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      32     b        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      33     c        No Impregnation in                                                                          No Baking                                                       Processing Fluid                                                                            Processing                                      34     d        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      35     e        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      36     f        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      37     g        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      38     h        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      39     i        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      41     j        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      42     k        No Impregnation in                                                                          No Baking                                                                                                                        Processing Fluid                                                                            Processing                                      43     o        30% Chromic Acid                                                                            450° C., Baking                                                                Δ                                                               30 minutes                                      44     p        30% Chromic Acid                                                                            450° C., Baking                                                                Δ                                                               30 minutes                                      45     q        30% Chromic Acid                                                                            450° C., Baking                                                                Δ                                                               30 minutes                                      46     r        30% Chromic Acid                                                                            450° C., Baking                                                                Δ                                                               30 minutes                                      Convention-                                                                   al Examples                                                                   51     q        No Impregnation in                                                                          No Baking                                                                             Δ                                                 Processing Fluid                                                                            Processing                                      52     r        No Impregnation in                                                                          No Baking                                                                             Δ                                                 Processing Fluid                                                                            Processing                                      __________________________________________________________________________     Note 1: The evaluation of the zinc bath immersion test (molten Zn bath        containing 3% Al, 500° C., an AISI 316 sample thermal sprayed on       one surface and having dimensions of 5 mm × 30 mm × 100 mm)       was as follows:                                                               ◯: No corrosive peeling after 30 days' immersion                  : No peeling after 10 days, corrosive peeling after 15 days'     immersion                                                                     Δ: Corrosive peeling after 10 days' immersion                      

We claim:
 1. A method of manufacturing an immersion member for use in amolten metal bath, comprising;forming a thermal sprayed coating on theimmersion member, said coating comprising 1-50 wt % of tungsten boride,3-25 wt % of at least one of Ni, Co, Cr and Mo as a metal phase, and aremainder of tungsten carbide, impregnating a processing fluidcontaining chromic acid on said thermal sprayed coating to at least fillcracks and micropores on the thermal sprayed coating with saidprocessing fluid, and baking said thermal sprayed coating with saidprocessing fluid to produce a Cr₂ O₃ -B₂ O₃ system glass in at least thecracks and micropores of the thermal sprayed coating to thereby form theimmersion member without the cracks and micropores.
 2. A method ofmanufacturing an immersion member according to claim 1, wherein saidbaking is conducted at a temperature between 400° and 500° C.
 3. Amethod of manufacturing an immersion member according to claim 1,wherein said processing fluid contains at least one of ammoniummolybdate and sodium molybdate.
 4. A method of manufacturing animmersion member according to claim 1, wherein said thermal sprayedcoating contains 10-40 wt % of tungsten boride.
 5. A method ofmanufacturing an immersion member according to claim 1, wherein saidcoating consists essentially of 1-50 wt % of tungsten boride, 3-25 wt %of at least one of Ni, Co, Cr and Mo, and a remainder of tungstencarbide,
 6. A method of manufacturing an immersion member for use in amolten metal bath, comprising;forming a thermal sprayed coating on theimmersion member, said coating comprising 1-49 wt % of tungsten boride;1-30 wt % of at least one of chromium boride, molybdenum boride,zirconium boride and titanium boride, a total amount of said borides andtungsten boride being less than 50 wt %; 3-25 wt % of at least one ofNi, Co, Cr and Mo as a metal phase; and a remainder of tungsten carbide,impregnating a processing fluid containing chromic acid on said thermalsprayed coating to at least fill cracks and micropores of the thermalsprayed coating with said processing fluid, and baking said thermalsprayed coating with said processing fluid to produce a Cr₂ O₃ -B₂ O₃system glass in at least the cracks and micropores of the thermalsprayed coating to thereby form the immersion member without the cracksand micropores.
 7. A method of manufacturing an immersion memberaccording to claim 6, wherein said baking is conducted at a temperaturebetween 400° and 500° C.
 8. A method of manufacturing an immersionmember according to claim 6, wherein said processing fluid contains atleast one of ammonium molybdate and sodium molybdate.
 9. A method ofmanufacturing an immersion member according to claim 6, wherein saidthermal sprayed coating consists essentially of 1-49 wt % of tungstenboride; 1-30 wt % of at least one of chromium boride, molybdenum boride,zirconium boride and titanium boride, a total amount of said borides andtungsten boride being less than 50 wt %; 3-25 wt % of at least one ofNi, Co, Cr and Mo; and a remainder of tungsten carbide.